This invention relates to gas sensing. More particularly, it is concerned with methods and apparatus for measuring the concentration of NOx (oxides of nitrogen such as N.sub.2 O, NO, NO.sub.2) in a gas mixture.
Various techniques and apparatus have been developed for determining the concentration of NOx in a gas mixture, particularly gas mixtures including oxygen and nitrogen. Typically the electrochemical sensing of NOx and other gases has been based on the oxygen pumping principle, for example, as described in U.S. Pat. No. 4,770,760 to Noda et al. and U.S. Pat. No. 5,034,107 to Wang et al. The sensing apparatus employed require two sensing elements. One element senses only oxygen gas and the other element senses all the gases which contain oxygen, including the oxygen gas. Both sensing elements are exposed to the same gas mixture, and the difference between the sensing signals produced by these two elements is a measure of the concentration of NOx in the gas mixture. Since the NOx concentration is determined from the difference between two signals, the accuracy of measurement is determined by the relative values of the two signals. If the gas mixture contains a relatively low NOx concentration as compared with that of oxygen, the signal-to-noise ratio is small and an accurate determination of the NOx concentration is difficult.